Semiconductor devices may generally be formed by modifying silicon substrates using front end of the line (FEOL) processes and back end of the line (BEOL) processes. FEOL processes are used to form devices such as transistors using ion implantation and other techniques. BEOL techniques are used to create metallization that connects the various devices (e.g., the transistors) with each other and to external devices.
BEOL processing typically involves depositing conductive layers (e.g., interconnects) separated and insulated by dielectric materials. As semiconductor device sizes continue to shrink and device densities continue to increase, capacitance (both for resistive capacitive (RC) delay and power consumption) and cross-talk between interconnects becomes a greater concern. Silicon dioxide (SiO2) has long been a primary dielectric used in BEOL processing; however, silicon dioxide has a dielectric constant, k=3.9, which is too high for many applications. As a result, low-k (e.g., k<3.0) and ultra low-k (e.g., k<2.5) dielectrics are now being used.
Moreover, shrinking device sizes and increased device densities has led to the use of copper for interconnects due to its high conductivity and improved electromigration resistance. However, copper can readily diffuse into dielectrics and react with silicon, of which may lead to device performance and reliability issues. As a result, barrier layers surrounding the copper used in metallization are deposited to protect materials adjacent to the copper.
Atomic layer deposition (ALD) is a process used to deposit conformal layers with atomic scale thickness control during various semiconductor processing operations. ALD may be used to deposit barrier layers, adhesion layers, seed layers, dielectric layers, conductive layers, etc. ALD is a multi-step self-limiting process that includes the use of at least two precursors or reagents. Generally, a first precursor (or reagent) is introduced into a processing chamber containing a substrate and adsorbs on the surface of the substrate. Excess first precursor is purged and/or pumped away. A second precursor (or reagent) is then introduced into the chamber and reacts with the adsorbed layer to form a deposited layer via a deposition reaction. The deposition reaction is self-limiting in that the reaction terminates once the initially adsorbed layer is consumed by the second precursor. Excess second precursor is purged and/or pumped away. The aforementioned steps constitute one deposition or ALD “cycle.” The process is repeated to form the next layer, with the number of cycles determining the total deposited film thickness.
A major challenge is that ALD is surface sensitive. The quality of the deposited film and/or the ability to nucleate a reaction and/or the ability to deposit uniformly without pin holes across a variety of surfaces with and without topography/topology is largely dependent on the ability to form a uniform, adsorbed layer of the first precursor (or reagent) on the surface(s) of interest. Many ALD precursors readily adsorb (e.g., chemisorb) on hydroxyl (—OH) terminated surfaces such as silicon dioxide. However, low-k dielectric surfaces tend to be hydrophobic and as such have a much lower surface hydroxyl concentration. In contrast, these surfaces are generally terminated via hydrocarbon groups including but not limited to methyl (—CH3) and ethyl (—C2H5) groups, which do not readily react with most ALD precursors, and therefore do not serve as good binding sites for such precursors. Additionally, many low-k dielectrics rely on film porosity as a means of further reducing the effective dielectric constant. These materials present additional challenges as the ALD precursor can penetrate more easily into such exposed pores and poison the dielectric material. Moreover, device structures such as damascene structures used in copper interconnects contain a variety of surfaces in conjunction with topography making uniform nucleation and subsequent growth of conformal, uniform, pin-hole free ALD film difficult.
Thus, what are needed are techniques for enhancing and improving ALD nucleation on a substrate.